sclass _Activity {
  long started;
  O r;
  Thread thread;
}

static Set<Activity> tempActivity_list = synchroHashSet();

static <A> AutoCloseable tempActivity(O r) {
  new _Activity a;
  a.started = sysNow();
  a.r = r;
  a.thread = currentThread();
  tempActivity_list.add(a);
  ret new AutoCloseable {
    public void close() {
      tempActivity_list.remove(a);
    }
  };
}